Cut-off wheels of fused alumina-zirconia alloy abrasive grains

ABSTRACT

Circular abrasive blades, known as cut-off wheels, are made faster cutting and more durable by employing fused, fine grained, alumina-zirconia abrasive grits, containing 10 to 60% and preferably 20 to 50% zirconia, having an elongated shape corresponding to a bulk density for 25% zirconia material having a specific gravity of 4.35, in the 24 grit size, of not more than 135 pounds per cubic foot, as at least 25% by volume, of the abrasive in the wheel. This invention relates to the class of relatively thin grinding wheels used for cutting metal, such as for cutting the gates and risers from ferrous castings, known in the art as cut-off wheels, and in particular it relates to improving the performance of such wheels by the employment in such wheels of fused alumina-zirconia abrasive grits of particular composition and shape.

United States Patent Rowse et al.

[4 1 Nov. 4, 1975 [75] Inventors: Robert A. Rowse, Shrewsbury;

Hussain G. Lakhani, Holden, both of Mass.

[73] Assignee: Norton Company, Worcester, Mass.

[22] Filed: May 10, 1974 [21] Appl. No.: 468,799

Related US. Application Data [63] Continuation of Ser. No. 204,373, Dec. 2, 1971 abandoned.

[52] US. Cl. 51/298; 51/309 [51] Int. Cl. C09K 3/14 [58] Field of Search 51/298,,309

[56] References Cited UNITED STATES PATENTS 3,156,545 11/1964 Kistler 51/298 3,175,894 3/1965 Foot 51/298 3,181,939 5/1965 Marshall at al. 51/309 3,524,286 8/1970 Wohrer 51/298 3,607,162 9/1971 Bockstiegel et a1 51/309 FOREIGN PATENTS OR APPLICATIONS 878,433 8/1971 Canada 5l/298 OTHER PUBLICATIONS Federal Register, Dept. of Labor, Vol. 36, No. 105, Part II, May 29, 1971, pp. 10629, 10630.

Primary Examiner-Donald J. Arnold Attorney, Agent, or FirmRufus M. Franklin [57] ABSTRACT Circular abrasive blades, known as cut-off wheels, are made faster cutting and more durable by employing fused, fine grained, alumina-zirconia abrasive grits, containing 10 to 60% and preferably to zirconia, having an elongated shape corresponding to a bulk density for 25% zirconia material having a specific gravity of 4.35, in the 24 grit size, of not more than pounds per cubic foot, as at least 25% by volume, of the abrasive in the wheel.

This invention relates to the class of relatively thin grinding wheels used for cutting metal, such as for cutting the gates and risers from ferrous castings, known in the art as cut-0E wheels, and in particular it relates to improving the performance of such wheels by the employment in such wheels of fused a]uminazirconia abrasive grits of particular composition and shape.

4 Claims, No Drawings CUT-OFF WHEELS OF FUSED ALUMINA-ZIRCONIA ALLOY ABRASIVE GRAINS This is a continuation of application Ser. No. 204,373, filed Dec. 2, 1971 and now abandoned.

FIELD OF THE INVENTION Maximum Wheel Diameter Thickness 6" and smaller 3/16" Larger than 6" to 12'' H4" Larger than 12" to 23" 3/8" Larger than 23" H2" As used in this specification and claims, the term cutoff wheel (or synonymously cutting-off wheel) means wheels complying with the OSHA. Rules and Regulations published May 29, 1971, in the Federal Register.

BACKGROUND OF THE INVENTION The standard abrasive grain commercially employed in cut-off wheels for cutting-off ferrous metals, has been the standard or regular fused alumina produced by the electric arc furnacing of bauxite and having an alumina content of about 95%. Such abrasive is available in so-called strong blocky shapes, and in weaker, slivery shapes. The preferred grain for the cut-off wheels has traditionally been the blocky, strong shaped grain. When weak shaped grain is substituted in the wheels, the rate of cut, under very light, pressure cutting-off may be increased, but the durability of the wheels under all cutting conditions is reduced, resulting in an uneconomical grinding operation.

The use of microcrystalline fused alumina grits in cut-off wheels, as opposed to the essentially monocrystalline abrasive employed as the standard, has not achieved commercial success. Although more durable than the standard alumina grits, such grits result in wheels having a lower rate or cut than when the stan' dard alumina grit is employed, which is not sufficiently offset by increased durability for economical operation.

Recently the fused alumina-zirconia alloy abrasives have come on the market. Such abrasives are more tough and durable than prior art fused alumina abrasives and have been employed extensively in heavy duty snagging grinding applications such as the cleaning of steel billets and the heavy duty snagging of ferrous metal castings. Such abrasives have been employed, almost exclusively in the form of strong, blocky shaped grains, although recently they have been found to be useful in weaker shapes for the manufacture of coated abrasives.

Until the present invention, the alumina-zirconia alloy type abrasives have not been employed commer cially in cut-off wheels. I

SUMMARY OF THE INVENTION Significantly improved grinding efficiency has been achieved according to the present invention by employing weak shaped alumina-zirconia alloy abrasive grits in thermoset or infusible resin bonded cut-off wheels.

US. Pat. No. 3,181,939, discloses microcrystalline alumina-zirconia fused alloy abrasives containing from 10 to 60% zirconia. The finding of the present inven tion is that the more finely microcrystalline materials of this type, when produced in slivery or weak shaped grain significantly improve the grinding efficiency of thermoset resin bonded cut-off wheels in which they are incorporated.

Slivery or weak shaped grain of the desired size is produced by the rolls crushing of material of coarser grit size than required, followed by screening.

Best results are achieved with of the abrasive of the alumina-zirconia alloy type. However up to 75% by volume of the abrasive may be replaced by blocky shaped fused alumina, and improved results a chieved according to the present invention. Such substitution may be advantageous to reduce the cost of the wheel.

Although operative in the range of 10 to 60% 2irco nia, the preferred abrasive for the present invention contains from 20 to 50% zirconia. The microcrystallinity of the abrasive should be such that the diameter of the primary alpha alumina crystals is less than 40 microns. Such abrasive can be produced by the methods disclosed in US Pat. No. 3,181,939, and British Patent 1,233,997, teaches one method of rapid cooling to achieve the preferred microcrystallinity.

Rolls crushing of the abrasive results in elongated grits which pack more loosely than strong shaped, more equidimensional grits. Useful abrasive grits are those selected from a population, for a 25% zirconia product having a specific gravity of 4.35, in which the 24 grit sized material (US. Dept. of Commerce Standards) has a loose packed (or bulk) density of not more than pounds per cubic foot (2.1 grams/cc). For grain from the same population the bulk density decreases with finer grit sizes and increases with coarser grit sizes. For the purpose of this invention, a grain processing which results in a weight per cubic foot for the 24 grit fraction of not more than pounds per cubic foot will be satisfactory. Although a precise lower limit has not been determined, lower densities, in the 24 grit fraction, than 1 15 pounds per cubic foot, would not be practical, for the 25% zirconia material. For material containing other than 25% zirconia or for 25% material having a specific gravity other than 4.35, the maximum and minimum bulk density should be corrected by multiplying 135 by the ratio of the specific gravity of the material to the specific gravity of the 25% material (4.35).

To achieve best results, the abrasive should be bonded with a phenol-aldehyde thermoset bond, with high bond content and high abrasive content, the abrasive ranging from 48 to 56 volume percent, and bond from 22 to 50% by volume, with any remainder being pores.

SPECIFIC EMBODIMENTS OF THE INVENTION Example I An example of the new wheel is a 20 X 5/32 X 1 inch (20 inch diameter, 5/32 inch thick, 1 inch diameter hole) No. 24 1910.21 1, grit reinforced resinoid bonded product. A typical use for this wheel is for foundry cutoff work. This wheel. has the glass fabric con struction of standard commercial products.

Total .97 lbs.

.32 lbs.

1.25 lbs.

10.00 lbs.

19 cc of Carbosota, a neutral anthacene oil wetting agent sold by Barrett Division of Allied Chemical Company, is also used with the above ingredients. The mix is made in the conventional way. The powdered resin and pyrites are batched together before adding to the mixing machine.

The bulk density of the abrasive was 123 to 126 pounds per cubic foot.

2. Resin Coated RC and RC 22 glass fabric This wheel is molded with two layers of mix and one full diameter resin coated RC 35 glass in the center of the wheel. A piece of 14 inch diameter RC 22 glass is molded onto each side of the wheel. These two styles of glass are impregnated with a phenol-formaldehyde varnish. The coated fabric contains approximately 33% by weight resin and 67% by weight glass.

Style 35 glass has 3.5 X 3.5 mesh and a weight of 1 1.2 oz./sq. yd. Strength is approximately 560 pounds per inch width. Style 22 glass has 2 X 2 mesh and a weight of 7.1 oz./sq. yd. Strength is approximately 355 pounds per inch width.

The resin coating on the glass is dried to a volatile of 3 5%. In this form the fabric is limp but not sticky. Industrial Polymers & Chemicals Inc., Shrewsbury, Mass. is one supplier of style 35 and 22 glass.

3. A piece of thin paper is used on each side of the wheel in molding. This paper is used to help release the green wheel from the polyurethane surface on the steel mold plate.

B. Filling the Mold A inch diameter X 1 inch hole cut-off mold with rough sided steel plates is placed on a turn table. A piece of 20 inch paper is first placed on the bottom mold plate and then a sheet of 14 inch diameter X 1% inch hole RC 22 glass is placed on top of the paper. The turn table is started and 2.2 pound of mix is poured in and spread level. The full diameter RC 35 glass is added after which the second layer of 2.2 pounds of mix is added to the mold and spread level. Next the second sheet of 14 inch diameter RC 22 glass is placed on top of the second layer of mix. Finally the second layer of paper is placed on top of the 14 inch diameter RC 22 glass. To keep the wheel from warping it is important that the glass fabric discs be oriented. By oriented we mean that the warp direction of all glass discs be placed in the same direction.

The top plate is placed in position and the mold is given 1100 tons pressure (3.4 tons per sq. in.). The wheel is then stripped from the mold.

C. Baking Wheels The green wheels are set in a conventional method which is batt, wheel, batt, wheel, batt etc. approximately 25 wheels high. 650 pounds of dead weight is 4 put on top of the stack of wheels and batts during the bake to help produce a well knit structure of glass and abrasive mix. The wheels are baked at a top temperature of 160C for 6 hours.

For comparison purposes wheels were made identical to the above except that the abrasive was a blocky shaped standard or regular fused alumina A1 0 grit having a bulk density of 127 to 131 pounds per cubic foot. This is a standard commercial abrasive, prior to the present invention, for the tested application.

METHOD OF TEST A standard commercial cut-off saw was employed (Tabur), wheel speed was 2400 revolutions per minute, manual plunge cuts at 20 KWH indicated power were taken on 2 inch diameter nodular iron bars, five cuts per measurement.

RESULTS The invention wheel employing the rolls crushed zirconia-alumina alloy abrasive wore only 16.6% as much as the standard wheel while making twice the number of cuts. The time per cut was 4.1 seconds for the standard and 3.5 for the wheel of the invention.

Although the test results refer to 24 grit abrasive similar improvement is achieved in the useful grit range for cut-off wheels, normally from 12 grit through 36 grit sizes.

The weight per cubic foot values employed herein are determined by the following method. A calibrated cylindrical container 4 inches in diameter and 15 13/16 inches high, and having a volume as close as possible to 3447 cubic centimeters, is clamped to a heavy steel disc. The disc is mounted in a holder and plate and container are jolted by a cam at the rate of jolts per minute, with a free fall of 5/64 inches. A hopper is mounted above the top of the calibrated can. An amount of grain in excess of the calibrated content of the cylinder is placed in the hopper. A gate in the bottom of the hopper, uncovering an opening threefourths inch in diameter is opened and the jogging of the cylinder is simultaneously started. The cylinder is allowed to fill and is kept full during the jolting. The jolting is continued for 590 jolts, and the excess grain is scraped from the top of the cylinder with a straight edge. The weight per cubic foot is then determined by weighing the contents of the cylinder, which has previously been precisely calibrated as to volume.

What is claimed is:

l. A cut-off wheel of abrasive grains with a grit range of 12-36 bonded by an infusible resin bond, said wheel having maximum thickness as defined below:

Wheel Diameter Maximum Thickness 6 inches and smaller 3/16 inch larger than 6 inches to 12 inches l/4 inch 12 inches to 23 inches 3/8 inch larger than 23 inches 1/2 inch;

said wheel characterized in that at least 25% by weight of the abrasive content is a fused alumina-zirconia alloy abrasive having a zirconia content between 10 and 60% by weight, an average primary alpha alumina crystal diameter less than 40 microns, and an elongated shape such that its bulk density in the 24 grit size range is in the range of from:

115 times s.g./4.35. to 135 times s.g./4.35, pounds per cubic foot, where s.g. is the specific gravity of the abrasive grit material, said wheel containing from 075% by weight of blocky fused alumina grit.

by volume of blocky fused alumina grits.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIQN Patent No. 3,916,583 Da ed November 4, 1975 lnventofls) Robert A. Rowse and Hussain G. Lakhani It is certified that error appears in the aboveidentified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 65: "No.24l9l0.2ll" should read Column 4, line 51: after "grit" the word size should be inserted.

Signed and Scaled this A tres t:

RUTH C. MASON C. MARSHALL DANN Arresting Officer (ommixsz'uner oj'Parenls and Trademarks 

1. A CUT-OFF WHEEL OF ABRASIVE GRAINS WITH A GRIT RAGE OF 12-36 BONDED BY AN INFUSIBLE RESIN BOND, SAID WHEEL HAVING MAXIMUM THICKNESS AS DEFINED BELOW:
 2. A cut-off wheel as in claim 1 in which the fused alumina-zirconia grit has a zirconia content of between 20 and 50 weight percent.
 3. A cut-off wheel as in claim 1 in which the bond is a phenol-formaldehyde resin, and the zirconia content of the abrasive is 25 weight percent.
 4. A cut-off wheel as in claim 3 including up to 75% by volume of blocky fused alumina grits. 